A signal processing technology for processing an input signal using a converter device and obtaining a target output is known. For example, a noise suppressing technology exists. It suppresses noise in a noisy signal and outputs an enhanced signal. Here, the noisy signal is a signal in which noise is superposed on the target signal. The enhanced signal is a signal in which the target signal is emphasized. A noise suppressor which suppresses noise superposed on a target speech signal is used for various audio terminals such as a cellular phone or like.
As this kind of technological example, patent document 1 discloses a method to suppress noise by multiplying the suppression coefficient smaller than 1 to an input signal. Patent document 2 discloses a method to suppress noise by subtracting presumed noise directly from a noisy signal. Patent document 3 discloses a noise suppression system which can realize the sufficient noise suppression effect and the small distortion in the enhanced signal, even when a condition that noise is sufficiently small compared to a target signal is not satisfied. Patent document 3 assumes a case when the characteristics of noise mixed in the target signal is known to some extent beforehand. The technology described in patent document 3 suppresses noise by subtracting noise information recorded beforehand from a noisy signal. Here, noise information is information about characteristics of noise.